This application claims the priority of 197 53 696.4, filed Dec. 3, 1997, the disclosures of which is expressly incorporated by reference herein.
The invention relates to a device and a method for the generation of extreme ultraviolet and x-ray radiation, in which the medium that emits the radiation is a plasma. A preferred field of use includes applications that require x-ray light around the 10 nm wavelength range, as is the case, for example, in EUV projection lithography around the 13 nm spectral range, where compact, cost-efficient and long-life x-ray light sources are required. An additional field of use includes x-ray analysis methods such as photo electron spectroscopy or fluoro-x-ray analysis, which utilize the spectral range of soft x-ray radiation, and which can be realized on a laboratory scale if a compact source is available. Furthermore, the method and device can be utilized for the characterization of x-ray optics or x-ray detectors.
The use of a plasma as a source for x-ray light is known. For this purpose, for example, plasma focus discharge, Z-pinch discharge, capillary discharge or the gas-puff are used.
In this way, a plasma is generated between two sheet electrodes during the z-pinch discharge, for which two operating methods are available for voltage pulse selection. On the one hand, it enables an operation in which the gas discharge ignites on the surface of the insulator. This results in considerable wear of the insulator. On the other hand, the current pulses can also be selected so that the gas discharge ignites in the entire available volume between the anode and cathode, which is limited by the insulator. Thus, at the start of the plasma ignition, one or more fine plasma channels can be found near the insulator and the electrodes, which are burned off as a result. Because of the intrinsic magnetic field of the current that flows at that time, the plasma channels are compressed into a single plasma channel (pinch effect) along the axis of symmetry of the electrode arrangement. Once the current is extinguished, the plasma spreads again up to the insulator, which also involves a burn-up of the insulator material. A further disadvantage of the Z-pinch discharge is the fact that intrinsic breakthrough operation is unsuitable for the formation of effective plasma emission. Typically, the Z-pinch is only used for a single pulse operation, and thus the yield of x-ray light is low. State of the art for the repetition rate, i.e., the rate for the build-up and breakdown of the x-ray light emitting plasma, is typically 20 pulses per second maximum.
If a gas puff is used to produce x-ray light emitting plasma, gas is admitted batch-wise through a suitably located opening in the insulator or the electrodes into the space between the anode and the cathode. The plasma build-up then occurs with the gas between the electrodes. Moreover, gas puff, Z-pinch and plasma focus discharge are carried out under high discharge currents ( greater than 100 kA) and in the range of several kilojoules for the electrically stored energy. They are thus optimized for a plasma, which emits x-ray light in this spectral range of several nanometers (hard x-ray radiation).
For longer wave radiation in the range of approximately 10 nm to approximately 50 nm, gas discharges with low currents or with a lower transferred energy per pulse are used.
At the same time, the capillary discharge is used, for example, which produces the same disadvantages with respect to wear mentioned above. As one of the variations of the embodiment a capillary discharge should be mentioned here, where use of the insulator for the ignition of the plasma is mandatory. Characteristic of this type of discharge is the vaporization of the insulator material that is subsequently converted to the plasma state. Thus, the insulator burn-off is also high. At the same time, all of the discharge types referred to are operated with an operating point on the right branch of the Paschen curve, with the plasma heated up by the pinch effect at the same time in order to attain the required temperature for the x-ray light emission.
The use of pseudo spark switches is known by way of prior art, which, for example, discloses one of the embodiment variations as a multi-channel pseudo spark switch in DE 39 42 307 A1. These are characterized in that the plasma is not ignited in contact with the insulator or the electrode surface, and that their overall life is therefore long. Their high repetition rate is also advantageous. The disadvantage of the switches however is that they are operated with respect to the type of gas, gas pressure and the current pulses in such a way that a plasma is formed in them which has only a low energy density, which rules out effective emission of EUV or x-ray radiation. Thus, for the emission of EUV or x-ray radiation a device would be advantageous in which the advantages of the Z-pinch discharge are combined with the advantages of the pseudo spark switch.
The object of the invention is to provide a device and a method for the generation of x-ray radiation and/or extreme ultraviolet (EUV) radiation from a gas discharge, with the insulator not exposed to any wear, and during which repetition rates up into the kHz range are possible, and at which suitable parameters for electrode geometry, gas pressure and type as well as current pulses for a plasma may be selected that can produce sufficiently high energy density or temperature so that an effective emission of soft x-ray radiation is achieved.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.